Exploratory Development Of Pathogenic Fungi-Fusarium Stoveri Against Alligator Weed (Alternanthera Philoxeroides)

Alligator weed, Alternanthera philoxeroides (Mart.) Griseb is regarded as an important alien noxious weed, which has become a major invader of both aquatic and terrestrial habitats since the eighties of the 20th century. The biological invasion of the pernicious weed not only leads to the extinction of native species, but also reduces the biodiversity where the weed occurs. Successful biological control of the deleterious weed has been achieved by using its natural enemies and plant pathogens have been integrated to manage the weed. In this study, we displayed one new bioherbicidal fungi—Fusarium stoveri antagonisted to alligator weed and searched for its biological potentiality, applied technique and processing of formulation.Several potential fungal bioherbicidal strains of F. stoveri caused the leaves wither, brown or etiolation and stems rot were isolated from the native alligator weed. The results revealed the suitable temperature of mycelial growth and conidia production was within range of 25-28℃, the optimum temperature was at 28℃. Great difference of the quantity of conidia production and the ability of pathogenicity were existed among the nine isolates exhibited stronger pathogenicity to alligator weed. The strain 32-6 displayed the best conidia production, which was selected as the potential bioherbicidal candidate based on the mycelial growth rate, conidia production and pathogenicity to alligator weed. Host-range tests of the strain 32-6 indicated that the pathogen didn't infect the 42 tested agricultural crops including several weeds species.We revealed the factors of pathogenicity using different inoculated spores concentration, volume, drop size under the greenhouse and field conditions, respectively. Results indicated that existed positive correlation between spore pathogenicity, inoculated concentration and/or relative humidity both of greenhouse and field conditions. There was no correlation between spore pathogenicity and inoculated drop size under the same inoculated volume (50-100mL/m2) regardless of greenhouse and/or field conditions. The key effective factors of pathogenicity of F. stoveri isolate 32-6 was the conidia concentration and environmental relative humidity and the optimum inoculum concentration was 1×l07spore/mL.To produce in large quantity spore, this study was aimed at discovering spore product condition of the cheap material liquid medium and solid medium. Based on spore production under different conditions (media, temperature, initial pH and initial inoculum concentration), an optimum spore productive condition for mung bean medium (MMB) was 28℃, initial pH8.0,5 d and 1%(V/V) of initial inoculum (1×106spore/mL).Furthermore, the optimum spore productive conditions for solid medium was 84.4% wheat bran,15.6% rice bran, initial water content 63%-65%, temperature 28℃, pH8.0,9 d, upon 50:1 (g/mL) initial inoculum (1×106spore/mL).Results of environmental factors for conidial germination of F. stoveri isolate 32-6 under the controlled conditions indicated that the optimum condition of conidial germination was 28℃, pH8.0. Relative humidity (RH) was the main inhibitive factor on conidial germination. At RH80%, the conidia couldn't germinate, and moreover, at RH95% and RH100%, the conidial germination rates were 23.4% and 46.3%, respectively. The results also indicated that the illumination had no significant effect on the conidial germination.To understand the joint application between herbicides and bio-fungi (F. stoveri 32-6), the effectiveness of mycelial growth, germinal tube elongation and conidia germination of F. stoveri isolate 32-6 with five widely used herbicides (Fluroxypyr-mepthyl, Paraquat, Fomesafen, Glyphosate and Tribenuron-methyl) was investigated using solid culture containing different fold of the concentration recommended for field spray under controlled condition and field experiment. The results indicated that Fluroxypyr-mepthyl had the smallest inhibition on the germinal tube elongation and mycelial growth among the five herbicides used at 50mg/L(1/4 of normal recommended concentrations).Field experiment result indicated, that the effectiveness of mixed formulation (1×107spore/mL plus 50 mg/L Fluroxypyr-mepthyl) was more than the recommended concentration of Fluroxypyr-mepthyl (200mg/L). Furthmore, the effectiveness was close to the 200mg/L Fluroxypyr-mepthyl when using mixed formulation (1×107spore/mL plus 25 mg/L Fluroxypyr-mepthyl). It was confirmed that it is possible to choose compatible herbicide (Fluroxypyr-mepthyl) at normal rates for enhancing the efficacy of F.stoveri isolate 32-6 in control of alligator weed and reducing the input of chemical herbicide.Employing scanning electron microscope (SEM) and transmission electron microscope (TEM), we observed the pathohistology of alligator weed infested by the conidia of F. stoveri isolate 32-6. The result indicated that the leaf stoma was one of the main penetrative channel and this pathogen could destroy leaf chloroplast.